Question

how do the atmospheric conditions near the beginning of Precambrian time contrast with atmospheric conditions that are present now which organism is largely responsible for this change

Answer 1

Note: The Precambrian time was the earliest geologic ages.

Then: Heavy with CO2 and water vapors, oxygen levels rose during Archeozic Era(3800 to 2500 ma) by photosynthesis. Singled celled microorganisms like bacteria and viruses flourished.
Now: Impact found in present day South Africa and Canada. Many fossil evidences were found. I don't have much to say on this one..
I hope this helped.